The two parts can, for example, be in form of a port plate and a cylinder drum of an axial piston machine or in form of a side plate and a rotor of a vane type machine. Another possibility is a slide shoe or slipper of an axial piston pump sliding over a swashplate. The two parts together limit a pressure chamber the volume of which may vary during an operation cycle of the machine.
The two parts have to be pressed together with a certain force. This force must be big enough to ensure a tightness, i.e. to keep leakages as low as possible. On the other hand, this force must not be too high to avoid friction losses and wear. In most cases the force pressing the two parts together is at least partly generated by hydraulic pressures. These pressures must be balanced.
When during operation the first land is worn, for example by abrasive wear after a certain working life, the pressure balance is disturbed. In some cases there is the risk that the force pressing the two parts together increases. In this case the leakages decrease and the further wear increases dramatically so that a necessary maintenance of the machine becomes more complicated.